What is "SDMK I" model? As in Marushin UZI/MP40/SDMKI cart?

Marushin cartridges typically used in UZI and MP40 have a third marking on the box: "SDMK I".

Do you have any idea what "SDMK I" would be?
Existing modelgun that I can't decipher, or something that was planned but not released?

Hmmm, Marushin did release an airsoft model of "Sturm Ruger Mk I SD", which was a silenced version of a Ruger Mk I:
http://www.wgcshop.com/wgc2008/main/product_detail1.php?search_From=category&item=MS-PS-MKSDB&search=special&rs=Marushin%20Handguns&catid=&cat=

This suggests that they might have planned a modelgun version in the past :-/

The only other Ruger Mk I model (but not silenced) that I know of was from CMC. Maybe Marushin wanted to take over the molds but did not succeed.